The present invention relates to a method for batchwisely calcining and activating infusibilized spherical pitch beads in a two-stage fluidized bed, the beads having been obtained by molding a petroleum pitch or a coal pitch into spheres and infusibilizing thereof.
So far, carbon beads or activated carbon beads are manufactured by the methods comprising: (1) blending petroleum pitch or coal pitch with a viscosity-adjusting agent; (2) melt-molding the blend into spheres; (3) extracting the agent by a solvent from the molded; (4) infusibilizing the extracted; (5) calcinating the infusibilized to have carbon beads and (6) activating the beads into activated carbon beads.
Infusibilization of the pitch beads is performed at a temperature not higher than 400.degree. C. in the presence of an oxidizing gas, such as O.sub.2, O.sub.3, SO.sub.3, NO.sub.2, an oxidizing gas diluted with air or nitrogen, or air, oxidizing and thermally infusibilizing the pitch beads. Then, the infusibilized pitch beads obtained are calcinated at a temperature not lower than 600.degree. C. in an inactive atmosphere to give carbon beads. Further, by activating the infusibilized spherical pitch beads or the spherical carbon beads with an activating gas, mainly consisting of nitrogen and steam, at a temperature of 800.degree. to 1,200.degree. C., the activated spherical carbon beads are obtained.
As an apparatus to produce spherical carbon beads or spherical activated carbon beads, a rotary kiln system or a one stage batchwise fluidized bed system have been widely used. However, in the rotary kiln system, there are following problems: namely, too large size of apparatus due to a poor volume efficiency of the furnace, the resultant long reaction time, low efficiency of operation due to a complicated mechanism to prevent inflow of air to the kiln during a high temperature activation and short life of the apparatus due to thermal strain and oxidation of its materials. On the other hand, in the one-stage batchwise fluidized bed system, since the difference between the both temperatures at the start of carbonization and at the end of activation is as large as at least 500.degree. to 600.degree. C., there are large problems that it is difficult to select a furnace material which is durable to high temperature and heat-shock and it is difficult to continue the operation for a long time period and to decrease the reparing cost, due to the short life of the furnace material. That is, as the apparatus material, an expensive heat proof and anticorrosive one has been used, however, in order to further decrease the repairing cost and prolong the stable operation, the temperature-raising rate from the end of infusibilization of pitch beads to the start of activation of infusibilized pitch beads had to be decreased, for instant, upto 50.degree. C./hr and further, the activating reaction temperature had to be as low as possible. Therefore, there is a problem that the reaction takes necessarily a long time. Accordingly, the system is not necessarily a satisfactory one.
On the other hand, there has been proposed a process for continuously activating granular carbonized coal particles in a plurality of interconnected fluidized beds supported on a horizontal gas distributor plates having tuyeres projecting above the upper surface thereof (refer to U.S. Pat. No. 3,976,597). However, in a horizontal-type apparatus using for this process, a plurality of compartments are connected horizontally and the activating gas is introduced into each of compartments in parallel. Therefore, it is difficult for such an apparatus to decrease an area for installation thereof and to decrease the necessary gas volume and the heat loss. In addition, since the fluidized particles are transferred to the next compartment by overflowing upon each outlet, the distribution of retention time of particles in the fluidized bed is spread resulting in lowering of uniformity in the product quality.
Further, there has been proposed a chamber for forming fluidized beds of finely divided solid such as iron ore fines, containing upper and lower beds supported on horizontal perforate partitions, the chamber having an overflow pipe connecting the both beds (refer to BP 880,792). However, also in this apparatus, the fine solid is transferred from the upper bed to the low bed through the overflow pipe and as a result, the distribution of retention time of fine solid in the fluidized bed is spread resulting in lowering of uniformity in the product quality.
As a result of the present inventors' extensive study on a method and an apparatus to activate infusibilized spherical pitch beads easily and in a high yield in order to overcome the above problems, they have found a new method for batchwisely calcining and activating the pitch beads in an upper stage and a lower stage fluidized beds formed on respective perforated plates, comprising the steps of calcining the pitch beads in the upper stage at 600.degree.-800.degree. C. with a heated exhaust gas from a space for the lower stage, discharging activated carbon beads of the previous batch from the space for the lower stage, stopping the fluidization of the upper stage by guiding the exhaust gas from the space for the lower stage to the space for the upper stage through a bypass line so that the calcined beads can fall down to the space for the lower stage through the holes of the perforated plate of the upper stage, and activating the falling calcined beads at 800.degree.-1200.degree. C. with a heated calcining and activating gas which has a rising gas velocity maintained in a certain limited range.
By this new method it is possible to increase their yield per unit time, to reduce tower diameter of the furnace and to reduce the energy cost due to a reduction of the reaction time. On the basis of these findings, the present inventors have attained the present invention.
That is, the first object of the present invention is to provide a method to produce the spherical activated carbon beads with a good productivity.
The second object of the present invention is to provide a method for producing the spherical activated carbon of a uniform quality at a relatively low cost.
The third object of the present invention is to provide a method for producing spherical activated carbon beads, which method is able to use a more compact apparatus with the same productive capacity as compared to a conventional method.
The fourth object of the present invention is to provide an activating furnace for activating infusibilized spherical pitch beads, which is more compact with the same productive capacity than a conventional apparatus.